Field of the Invention
The present invention relates to lithium-based energy storage devices generally, and, in particular, to negative electrode active materials for lithium-based energy storage devices.
Description of the Related Art
Energy storage devices utilizing lithium ions for energy storage applications have one of the highest energy storage capacities of all rechargeable batteries and are widely used today. Lithium ion capacitors and lithium ion batteries are two basic types of energy storage devices, whose negative and positive electrode materials store/release lithium ions through different processes. Lithium ion batteries are typically employed where charge/discharge times are in a range of tens of minutes or longer. Lithium ion capacitors are characterized by a much faster charge/discharge rate compared to lithium ion batteries, e.g., several seconds. Moreover, lithium ion capacitors are expected to be cycled at least ten thousand charge/discharge cycles without a significant loss in capacity, such as 20%, whereas lithium ion batteries are generally required to be stable over one or two thousand cycles.
Cycling stability of a lithium ion capacitor generally relies on the negative electrode active material. During a cycling process, a faradaic process is associated with the negative electrode, which may result in a structural change of the negative electrode active material. Here, the faradaic process means that the charge transfer process is a result of an electrochemical reaction. The positive electrode of the lithium ion capacitor is, in theory, stable over unlimited cycles since the positive electrode material (e.g., activated carbon) does not experience any significant chemical or physical change during the cycling process.
In a conventional lithium ion capacitor, a graphite material is used as the negative electrode active material to host/release the lithium ions through faradaic lithium insertion/extraction processes, while a highly porous carbon is used as the positive electrode material to provide a high surface area for adsorption/desorption of ions. The graphite material has the advantage of a relatively high lithium diffusion rate that provides a high rate capability for the lithium ion battery, while a small volume change is necessary to achieve long cycling stability. However, a graphite material has a low energy storage capacity (e.g., 374 mAh/g) and a low volumetric energy storage capacity (e.g., 818 mAh/cm3 assuming that the density of graphite is 2.2 g/cm3), limiting the storage capacity of the lithium ion capacitor.
Tin has a good lithium diffusion rate and has higher theoretical lithium storage capacity (e.g., 994 mAh/g or 7254 mAh/cm3) than that of the graphite material. Tin has been studied extensively as the negative electrode active material for lithium ion batteries. However, tin in a lithium ion battery tends to have poor cycling stability because of the large volume expansion/contraction of tin in the lithium insertion/extraction process. The volume change of a tin particle can be as large as 300% during the lithium insertion/extraction process, which can result in the formation of cracks in the electrode film and the loss of electrical contacts among tin particles during cycling. This will result in a decrease of energy storage capacity since electrons cannot be transferred to the isolated tin particles. The cycling stability may become even worse at a high charge/discharge rate since there may not be enough time for the tin particles to release the lithium ion-induced volume change stress resulting in a mechanical breaking of the tin particles. Efforts have been made to improve the cycling stability by limiting the cycled voltage of a tin electrode to 0.3V to 0.7V (relative to Li/Li+ redox potential) by Hosono et al. (Eiji Hosono, Hirofumi Matsuda, Itaru Honma, Masaki Ichihara, and Haoshen Zhou; High-Rate Lithium Ion with Flat Plateau Based on Self-Nanoporous Structure of Tin Electrode; Journal of The Electrochemical Society, vol. 154, no. 2, (Dec. 27, 2006), pp. A146-A149). However, this research showed that the tin electrode is stable for only 20 cycles. It is questionable whether the tin electrode can survive thousands of cycles considering the continuous volume change of the tin particles and the absent a stress-buffer component in the tin electrode material.
To improve the cycling stability of the tin, a stress-buffer component can be incorporated to reduce the volume change stress so that the electrical contacts among the tin particles can be maintained during cycling. The stress-buffer component generally is a material that is stable during the charge/discharge cycling. It may be electrochemically-inert or may experience a different volume change profile than tin during cycling. For example, tin oxides with various compositions and structures have been reported as negative electrode active materials for lithium ion batteries. In these oxides, tin oxide will be converted irreversibly into tin and lithium oxide during the first cycle, and the lithium oxide will act as a stress-buffer component since it can be electrochemically-inert during cycling. These tin oxides, however, are tested against lithium in half cells with a limited number of cycles, for example, less than 100 cycles. Here, a half cell contains metallic lithium as a negative electrode active material and a tin oxide as a positive electrode active material with a lithium salt dissolved in a non-aqueous solvent as an electrolyte. Moreover, these mixed oxides generally show a large irreversible capacity loss during the first cycle mainly as a consequence of the oxide conversion (for example tin oxide is converted to tin and lithium oxide) and the solid-electrolyte interface (SEI) layer formation, which makes it impractical for them to be used directly in a lithium ion battery. Furthermore, it is questionable whether tin oxide-based materials can be used for high rate applications. To be used as a negative electrode active material for high rate applications, a material generally needs to have both good electrical conductivity and good lithium ion diffusion rate. Unlike tin, tin oxide is not electrically conductive and a lithiated tin oxide contains lithium oxide, which is not electrically conductive. The presence of the lithium oxide may slow down the rate performance of the tin particles since it is not electrically conductive. In one specific example, V1/2Sb1/2SnO4 used as a negative electrode active material for lithium ion batteries was reported by Reddy et al. (M. V. Reddy, G. V. Subba Rao, B. V. R. Chowdari; Nano-(V1/2Sb1/2Sn)O4: High Capacity, High Rate Anode Material for Li-ion Batteries, Journal of Materials Chemistry, vol. 21, (Feb. 24, 2011), pp. 10003-100011). The V1/2Sb1/2SnO4 had a relatively poor cycling stability when cycled in a half-cell with lithium as the negative electrode. The cycling stability was better after the oxide particles was milled using high energy ball milling into nano-scaled particles. However, the cycling stability was only demonstrated within 100 cycles. It is questionable if these nanoparticles can be stable over thousands of cycles. Moreover, the V1/2Sb1/2SnO4 has been evaluated at a charge/discharge rate of 3.5 C (about 17 minutes), which is still slow for high rate applications. Here, C is capacity of a battery measured in amp-hour. The rate performance of the V1/2Sb1/2SnO4 at a much higher rate such as 40 C (90 seconds) is questionable. Furthermore, the first irreversible capacity loss is about 1200 mAh/g, which is much higher than the obtained reversible capacity (about 580 mAh/g), which makes it impractical to use V1/2Sb1/2SnO4 directly as the negative electrode active material in a full cell because of the limited source of lithium in a full cell and the difficulty in balancing the amount of positive electrode active material. Furthermore, the high energy ball milling process is not practical for industrial scale production. It is necessary to develop a different process to make V1/2Sb1/2SnO4 nanoparticles that can be easily scaled up.
An energy storage device using a tin oxide-based negative electrode with a cycling stability greater than 5,000 cycles has not been reported. Therefore, a tin oxide-based negative electrode active material providing a good capacity and an excellent cycling stability at a high charge/discharge rate is needed for energy storage devices.